A fuel cell stack is prepared by connecting a plurality of unit cells. Each unit cell consists of a membrane electrode assembly (MEA) sandwiched between two gas diffusion layers (GDLs). The unit cells are separated by separators, and the MEA consists of catalyst layers of an anode and a cathode with an ion-conducting polymer electrolyte membrane between them. As the fuel cell stack is operated, degradation occurs in one or more unit cells for various reasons, resulting in degraded performance of the stack. The degradation of the unit cell occurs in the catalyst layer, a catalyst layer support, the electrolyte membrane, the separator, or the like.
At present, when degradation of stack performance occurs, the voltage of each cell is detected to detect a degraded cell and the stack is disassembled after stopping operation to analyze the cause in the cell. As such, since the disassembly of the fuel cell stack is necessary for evaluation of degraded unit cells and analysis of the underlying cause with the existing method, a method allowing evaluation of the degradation of unit cells in the fuel cell stack without disassembly of the stack is needed.